Foam fluid is widely used in the oil-gas field development field at home and abroad, and is evaluated as an intelligent fluid which is high in performance, environment-friendly, low in cost and low in damage. Foam fluid is applied in the whole process of oil-gas field development, from drilling fluid, completion fluid and fracturing fluid for drilling engineering, to an oil-displacing agent, a profile control agent, a water shutoff agent and an acidifying agent for oil production engineering, and even sand cleaning fluid and flushing fluid for workover treatment. Furthermore, foam fluid has an irreplaceable excellent effect for a plurality of formations with low permeability, low pressure and low saturability. A large number of oil and gas field production applications prove that the application of foam fluid can improve oil and gas production, protect oil and gas reservoirs and reduce production costs.
Foam fluid has been applied to oil-gas field development for over 50 years, however, there are still some problems for improvement of the foam application process, wherein stability of foam is a primary bottleneck that hinders the foam development. Foam is a thermodynamics unstable system and can finally break and disappear. The unstability of foam in formations is mainly represented by coalescence of bubbles in foam, precipitation of liquid in foam, and break of foam.
In recent years, there are mainly two ways to improve the stability of foam in the oil-gas field development field. The first way is to increase the viscosity of base fluid, so as to reduce the precipitation speed of foam and prolong gas-liquid separation time, which is mainly achieved by adding vegetable gelatin, artificially synthesized polymers, protein and crosslinking frozen gel to foam. However, the defect of the way is that due to the fact that insoluble residues and unbroken gel constituents are inevitable in vegetable gelatin and polymers, a pore throat structure in formations can be blocked to cause damage to formations, and further affect the productivity of oil-gas wells. For example, the Chinese patent document CN101805600A (application number: 201010150239.9) discloses a frozen gel fracturing fluid suitable for coalbed methane reservoirs. The frozen gel fracturing fluid consists of, by mass, 0.3-0.5% of nonionic polyacrylamide, 0.014-0.04% of zirconium oxychloride (ZrOCl2) which serves as a cross-linking agent, 0.01-0.12% of hydrochloric acid which serves as a pH modifier, 0.06-0.12% of gel breaker and balance of water, wherein the gel breaker is a redox system consisting of ammonium persulfate and sodium sulfite by a mass ratio of (1.0-3.0):1, and can break frozen gel at a low temperature. The frozen gel fracturing fluid disclosed in the patent document is adjustable in gelation time and gel breaking time, and has the advantages that low-temperature crosslinking speed is high, viscosity is high, filtration loss is low, gel breaking is complete, and gel breaking liquid is free of residue and can flow back easily. The frozen gel fracturing fluid can effectively improve coalbed methane productivity. However, in actual production, a coalbed adsorbs a large number of polymer molecules to cause damage to formations; besides, due to the uncontrollability of the construction process, a cross-linked polymer solution and the gel breaker cannot be fully mixed, which can cause non-uniform gel breaking, and then formations can be further damaged.
The second way of improving foam stability is to improve the mechanical strength of a foam liquid film, so as to improve the impact resistance and disturbance resistance of the liquid film, and further reduce foam break. The second way is mainly achieved by adding a particle foam stabilizer to the foam. For example, Chinese patent document CN102746841 A (application number: 201210223060.0) discloses a composite foam system containing nano-particles for an oil-gas field and a preparation method thereof. The composite foam system consists of, by mass, 0.3-0.5 part of anionic surfactant, 1-1.5 parts of modified silicon dioxide nano-particles, 0.03-2.3 parts of countra-ion salt and 100 parts of water. The components are mixed according to a certain ratio and stirred with a magnetic stirrer, and then left standstill. The mixture is stirred rapidly with the Waring Blender method to obtain foams with high stability. The foams produced by means of the composite foam system has a longer half-life period than the foams stabilized by an ordinary surfactant, but has a larger foaming volume than the foams produced by adding a foam stabilizer. The composite foam system has a simple formula and preparation technology as well as high salt tolerance and temperature tolerance, can be adapted to underground complicated oil deposit conditions, does not pollute formations, can effectively block macropore channels and improve sweep efficiency, and has great application prospects in oilfield development application, especially foam displacement. However, the usage of silicon dioxide nano-particles which are high in manufacturing cost in the composite foam system is high, and therefore the economic feasibility of applying silicon dioxide nano-particles to oil-gas development engineering is low.